Not at all.
He had no sisters (that we are aware of).
Ian,
Offa's Dyke is a massive linear earthwork oughly following some of the current border between Wales & England. In places, it is up to 65 feet (20 m) wide (including its surrounding ditch) and 8 feet (2.5 m) high. In the eighth century it formed some kind of delineation between the Anglian kingdom of Mercia and the Welsh kingdom of Powys. It has been the subject of considerable research in recent years, dispelling many of the earlier understandings.
I thought everyone knew that!
But you're right, it was probably one Dyke too many for the continuation of Mercia and its Monarchy as it is now the seat of the Barnfather dynasty!
Sean